Section 5 — Mutagenesis 



A number of these suspected suppressor mutants 

 were tested, using transduction experiments, to 

 determine whether each was site-specific, locus- 

 specific, or non-specific. 



5.16. Genetic Analysis of Adenine — 1 Revertants 

 in the Fission Yeast Schizosaccharomyces 

 pombe. C. H. Clarke (Edinburgh, Great 

 Britain) 



Ultraviolet induced adenine — 1 auxotrophs 

 isolated by Prof. U. Leupold have been used in 

 reverse mutation studies with S. pombe. Adenine- 

 independent revertants of spontaneous, HNO2 

 and u.v.-induced origin have been obtained and 

 tested for the presence of suppressor mutations. 

 Revertants giving no adenine-requiring recom- 

 binants among 300-500 progeny ascospores, 

 after backcrossing to wild-type, are classified 

 as "reversions". 



One adenine-1 mutant, number 40, gives 

 spontaneous revertants which are mainly of a 

 suppressor type, while most HNO2 and u.v.- 

 induced revertants of this same mutant are 

 "reversions". Another temperature-sensitive 

 mutant, adn.-l, 199, gives revertants spontane- 

 ously and with HNO2 treatment which are 

 mostly "reversions", u.v.-induced revertants of 

 mutant 199 are however predominantly of the 

 suppressor type. Furthermore the appearance of 

 HNG"2-induced "reversions" of mutant 199 is 

 only weakly, that of u.v.- induced suppressor 

 revertants strongly, inhibited by the addition of 

 L-methionine to the minimal medium upon 

 which adenine-independent colonies are scored. 

 These two differences between HNO2 and u.v. 

 induced revertants of the same mutant may be 

 related. The results indicate an unusual example 

 of mutagen specificity for individual alleles. 



Full details and results will be submitted for 

 publication to Genetical Research in due course. 



5.17. The Use of the Suppressors of Methionine 1 for 

 the Study of Mutation in Aspergillus nidulans. 



Lorna J. Lilly (London, Great Britain). 



During a survey of spontaneous reversion 

 rates from nutritional requirement to protrophy 

 in Aspergillus, it was found that the change from 

 methionine requirement to prototrophy oc- 

 curred with exceptionally high frequency (approx. 

 IO- 5 ). 



These reversions from methionine requirement 

 can be shown to be due to suppressor genes, all 

 those so far tested being unlinked to the methio- 



ninei locus. The suppressors have various 

 visible (morphological) phenotypes ("A", "B", 

 "C", etc.). If each phenotype is determined by a 

 single cistron, these suppressors could be used for 

 the simultaneous visible scoring of forward 

 mutation in several cistrons. 



The genetics of some of the phenotypes which 

 occur with highest frequency are being investi- 

 gated. The results of crossing one strain of type 

 "A" with one of type "B" indicate that each 

 strain carries a single suppressor and that these 

 suppressors are unlinked. Crosses between 

 strains of type "A" show that at least two 

 unlinked loci can independently determine 

 this phenotype. Crosses between the other 

 phenotypes are in progress and the suppressors 

 are being located. 



If two unlinked loci control the "A" phenotype 

 this phenotype is not suitable for the visible 

 scoring of mutation in a single cistron. This 

 result may affect the interpretation of results 

 obtained by workers who are using this system 

 to study chemically induced mutation. 



5.18. Kinetic Studies of Chemically Induced Reverse 

 Mutations in Neurospora crassa. H. G. Kol- 

 mark and B. J. Kilbey (Edinburgh, Great 

 Britain) and S. Kondo (Misima, Japan). 



The experiments were designed to follow the 

 effects of chemical concentration and time of 

 treatment on mutation induction. Conidia 

 from the strain K 3/17 ad-3A, 387ol, were 

 treated with diepoxybutane (DEB), or ethyleno- 

 xide(EO) in aqueous solution. (1 )The temperature 

 was kept at 21° or 22°C during treatment. 

 Treatments with DEB were carried out for 

 varied periods with concentrations varying 

 from 1.0 x 10- 2 to 3.16 10" 1 m. At the 

 higher concentrations of DEB it was found 

 that the frequency of adenine reverse mutations 

 among survivors was determined by the product 

 of time and concentration (the "dose"). After 

 a short initial period the frequency is approxi- 

 mately a linear function of (cone. time) 2 . This 

 result suggests that the induction of adenine 

 reversions by DEB has a two-event mechanism. 

 It seemed possible that this mechanism was due 

 to the existence of the two reactive epoxygroups 

 in the molecule of DEB. However, similar 

 experiments with EO, which has only one 

 epoxygroup, gave the same kinetics. It therefore 

 seems more likely that the basis for a two-event 

 mechanism resides in the induced genetic change. 

 The monofunctional compound, EO, is by far 

 the more efficient when similar doses of the two 

 compounds are compared. Mutation frequency 



61 



